Implications of Receiver Plane Uncertainty for the Static Stress Triggering Hypothesis

Large earthquakes are followed by a decaying number of smaller earthquakes, called aftershocks, which are hypothesized to occur on subsidiary faults and fractures stressed by the mainshock. The percentage of aftershocks potentially triggered by this mechanism depends on the locations and orientations of the aftershock faults, and is susceptible to observational uncertainties in these parameters. We evaluate the percentage of aftershocks encouraged by stress changes for the 1997 Umbria-Marche, 2009 L’Aquila (Italy), and 2019 Ridgecrest (California) earthquake sequences using large, published aftershock data sets. While only ∼50%–65% of aftershocks are nominally encouraged by mainshock stress changes, observational uncertainties are large enough that any aftershock could be explained by random chance, or could have been triggered. We also test the common assumption that aftershock faults are oriented to maximize failure in the applied stress field. Contrary to this assumption, we find that aftershocks tend occur on planes oriented more consistently with pre-existing mapped faults. Accounting for aftershock fault plane uncertainties is critical when evaluating the percentage of potentially triggered aftershocks. Additional sources of uncertainty or other triggering mechanisms may be required to explain aftershock occurrence, especially in the immediate vicinity of the mainshock